bug#44047: current HEAD has been crashing on startup

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 20 Oct 2020 16:19:26 -0600 (MDT)
> From: Liam Stitt <stittl@cuug.ab.ca>
> cc: 44047@debbugs.gnu.org
> 
> (gdb) frame 1
> #1  0x0000555555597de8 in build_frame_matrix_from_leaf_window (
>      frame_matrix=0x555555d4a2b0, w=0x555555d42420) at dispnew.c:2531
> 2531          fill_up_frame_row_with_spaces (frame_row, window_matrix->matrix_x);
> (gdb) print frame_matrix->rows
> $1 = (struct glyph_row *) 0x0
> (gdb) print frame_matrix->nrows
> $2 = 0

OK, so this is the problem: the frame glyph matrix has no glyph rows
allocated to it.  The question is how did that happen?

To find out, please run Emacs under GDB, after setting a breakpoint in
adjust_frame_glyphs_for_frame_redisplay:

  (gdb) break adjust_frame_glyphs_for_frame_redisplay
  (gdb) run -Q -nw

When the breakpoint breaks, step through the code with the "next"
command.  The first couple of times when the function is called, it
should return immediately, here:

  if (!FRAME_LIVE_P (f))
    return;

In the call after those 2, it should allocate the glyph pools and the
2 glyph matrices, then call allocate_matrices_for_frame_redisplay, and
return here:

	  if (!FRAME_WINDOW_P (f) && pool_changed_p)
	    SET_FRAME_GARBAGED (f);
	  return;

And in the next call, it should in addition allocate the glyph rows,
here:

      else
	{
	  adjust_glyph_matrix (NULL, f->desired_matrix, 0, 0, matrix_dim);
	  adjust_glyph_matrix (NULL, f->current_matrix, 0, 0, matrix_dim);
	  SET_FRAME_GARBAGED (f);
	}

This last call's backtrace should look like this:

  (gdb) bt
  #0  adjust_frame_glyphs_for_frame_redisplay (f=0xf46948) at dispnew.c:2008
  #1  0x000000000041c905 in adjust_frame_glyphs (f=0xf46948) at dispnew.c:1828
  #2  0x000000000042cdfd in adjust_frame_size (f=0xf46948, new_width=80,
      new_height=37, inhibit=5, pretend=false, parameter=XIL(0x3690))
      at frame.c:819
  #3  0x0000000000427657 in change_frame_size_1 (f=0xf46948, new_width=80,
      new_height=37, pretend=false, delay=false, safe=true, pixelwise=false)
      at dispnew.c:5798
  #4  0x00000000004276aa in change_frame_size (f=0xf46948, new_width=80,
      new_height=37, pretend=false, delay=false, safe=true, pixelwise=false)
      at dispnew.c:5830
  #5  0x00000000004290e2 in init_display_interactive () at dispnew.c:6397
  #6  0x0000000000429561 in init_display () at dispnew.c:6449
  #7  0x00000000005a892b in main (argc=3, argv=0x7fffffffe538) at emacs.c:1995

Does this happen to you as I described, or does something different
happen?

Also, each time allocate_matrices_for_frame_redisplay is called,
please show the value of matrix_dim after the call:

  (gdb) p matrix_dim

and also show the value of f->total_lines each time
adjust_frame_glyphs_for_frame_redisplay is called:

  (gdb) p f->total_lines.

Finally, the last time adjust_frame_glyphs_for_frame_redisplay is
called, and goes on through all of its code, you should see non-NULL
value of the frame's glyph_matrix's glyph_rows before the function
returns.  Here's what I see:

  (gdb) p f->desired_matrix->rows
  $11 = (struct glyph_row *) 0xf50140
  (gdb) p f->desired_matrix->nrows
  $12 = 38

(In my case, the TTY window has height of 38 lines.)

If what happens on your system matches the above description, then
somehow the value of f->desired_matrix->rows is NULLified after this
initialization.  But if you get NULL as f->desired_matrix->rows in the
last call to adjust_frame_glyphs_for_frame_redisplay, or if Emacs
segfaults before it reaches that point, then something abnormal
happens during initialization, and we will next look at that.

Thanks.